C :: Append Data To Array Of Structures In Memory Then Put In File When Program Ends

I have a program that stores health information the user inputs, one person at a time. The program works perfectly with the exception of storing the data...I need to open a file and read what health data it has in it already, if any, but store the new changes, and appended data to the array of structures, to the data in memory. All of the information is only saved back in the file once the program terminates. I'm not sure how to go about doing this, so I am also not sure what to put in the function for "Save and Exit" that the user can choose in order to exit the program.

Code:

#include "stdio.h"
#include "stdlib.h"
#include "string.h"
struct strdatabase
{
char first[20];
char last[20];
char gender[2];

[Code]...

I tried playing around with this bit of code, but I'm not sure if I'm even on the right track.

Code:

FILE *fp;
fp = fopen ("students.txt","rb"); fseek(fp, 0, SEEK_END);
long pos = ftell(fp);
fseek(fp, 0, SEEK_SET);
char *bytes = malloc(pos);
fread(bytes, pos, 1, fp);
fclose(fp);